Such machines have at least one rotor and can also comprise a plurality of rotors which can also be components of an intermediately arranged transmission. If there is a plurality of rotors, the invention can be applied to each individual. When there is a single rotor, the drive rotor is permanently connected to the working machine rotor along a common rotational axis to form a shaft line.
The preferred field of application of the invention is the power range of at least 1 MW absorption power which differs basically in terms of the dimensions, the selection of materials and the use of significantly smaller assemblies.
As a result of the rectifying and, in the next step, the inversion within the converter to the output frequency or the desired working frequency, not only the working frequency but also harmonic and inter-harmonic frequency components are generated in the electrical feed signal of the motor.
If the Fourier analysis (rapid Fourier transmission) is also used, inter alia, to determine frequency components which are not integral multiples of the frequency of the feed signal, these components are referred to as inter-harmonics.
These harmonic and inter-harmonic frequencies in the electrical feed signal of the motor are applied to the mechanical system in the air gap of the motor as exciting torsional moments.
A Campbell diagram permits the running performance of a machine in a rotational speed range to be assessed through this synopsis of the rotational speed, exciter frequencies and natural frequencies. The X axis of the Campbell diagram or the abscissa represents rotational speed of the rotor of the machine under consideration. If an oscillation profile which is dependent on the rotational speed, for example a torsional oscillation of a rotor shaft, is transformed from the time domain to the frequency domain by means of Fourier transformation, these are represented in the Campbell diagram as a rising and a falling straight line plotted against the X axis, wherein the latter represent the rotational speed of the rotor. Orders (O1, O2, . . . ) of the Fourier transformation are then reflected in these straight lines which appear as center point beams and whose gradient is proportional to the respective ordinal number. The frequency f of the natural frequency of the rotor or the rotating part which is subjected to consideration is represented on the ordinate. The natural frequencies are represented as a tolerance band whose respective width arises as a result of the inaccuracy of the model formation and, if appropriate, other variants. As a result, the natural torsional frequency relates, unless stated otherwise, to the described tolerance band in all cases. The bandwidth of the tolerance band is already obtained from irregularities of the geometry due to unavoidable fabrication tolerances. A tolerance band is preferably assumed to be wide here such that a calculation directly includes various embodiments of the machine, with the result that these variants are also covered by the dimensions. Accordingly, the tolerance band can preferably have a certain lack of precision.
In addition, harmonic exciter frequencies are represented, which are represented as straight lines parallel to the abscissa if they are independent of the rotational speed. If the exciter frequency varies with the rotational speed, said frequency is represented as a rising or falling straight line through the origin. If the rotational speed of the machine is in a range in which the exciter frequency profiles intersect the tolerance band of natural frequencies, increased oscillation altitudes are to be expected.
Inter-harmonic exciter frequencies occur as V-shaped, symmetrical beams for output frequencies F1, F2, F3, . . . ; Fn in the Campbell diagram. Wherein F1, . . . Fi, . . . Fn are grouped into concentration ranges G1, . . . , Gi, . . . Gz, wherein Fi which are close to one another and which together form a common output point are combined in Gi.
The upper and lower limits of the concentration range G1, . . . , Gi, . . . Gz are defined by the intersection point of the lowest natural torsional frequency of the rotor with the two straight lines of the beam pair of the inter-harmonics of the first order of the respective concentration range G1, . . . , Gi, . . . Gz. The intersection point in the case of inter-harmonics always denotes the coordinates with the highest frequency with respect to the range of the tolerance band which is intersected by the inter-harmonics. Insofar as an excitation of the second and/or third natural torsional frequency is mechanically possible, this is to be taken into account in the same way (mutatis mutandis) as described above for the first natural torsional frequency.
If the harmonic and inter-harmonic exciter frequencies are represented together with the natural torsional frequencies of the mechanical system in a Campbell diagram (exciter or natural frequencies plotted against the rotational speed of the motor), it is seen that in the operational range of conventionally configured motors, intersection points of the natural torsional frequency which can be excited by the motor (usually the first natural torsional frequency) with the inter-harmonic exciter frequencies occur. A steady-state operation of the mechanical system at one of these intersection points of inter-harmonic excitation and natural torsional frequency leads to a state of resonance with high torsional oscillation amplitudes and therefore to high dynamic torsional stresses in the torque-transmitting line components. The consequences which possibly result from this, for example fatigue damage to the load of the line components, should be avoided.
Drives with converter-controlled electric motors have as a rule a frequency converter and an electrical synchronous motor or asynchronous motor. While the input frequency into the converter is embodied as a pure sinusoidal oscillation on the basis of the virtually perfect rotational movement of the energy generation assemblies which feed the power system frequency, the spectrum of the frequency analysis shows that the output from the converter has, in addition to the set point frequency, other frequencies which can lead to excitations of torsional oscillations. Such undesired secondary frequencies, which have been virtually impossible to avoid hitherto, are also referred to as harmonic or inter-harmonic exciter frequencies. The inter-harmonic exciter frequencies within the customary operating rotational speed range of the motor usually give rise to excitation of torsional oscillations of the entire drive, for example driven compressor trains or other turbo sets.
Insofar as there is no intermediate transmission in the mechanical train, the additional loading, caused by the excited torsional oscillation, can occur largely unnoticed. However, the undesired dynamic additional loading in the mechanical line components give rise to a considerably shortened service life owing to fatigue of components.
If a transmission is a component of the machine, within the transmission toothed engagement occurs to form a coupling between torsional oscillations and radial oscillations. As a result, the torsional oscillations in the transmission also have the effect of shortening the service life. In addition, undesired large radial oscillations and undesired increased noise emission (rattling of the transmission) occur.
The problem of undesired torsional oscillations can be frequently detected only by means of a dynamic measurement of the torsional moment. Such a measurement is usually not used for continuously monitoring a turbo line, and would only identify torsional resonances which are present but would not avoid the cause of their generation.